This invention relates to protective relay circuits for use in AC power distribution systems and more particularly to transformer differential relays.
As part of the protection scheme for AC power distribution systems, it is desirable to detect faults occurring within the transformers used in the system. Upon detection of a fault within the transformer, circuit breakers are operated to disconnect the input and/or the output of the transformer thereby protecting the transformer as well as those portions of the power distribution system connected to the transformer.
Transformer differential relays have been used in the past to detect faults within a transformer. Such relays operate upon the theory that, in an unfaulted transformer, the current I.sub.p in the input windings is equal to the current I.sub.s in the output windings multiplied by the output to input winding turns ratio R.sub.t ; that is, I.sub.p =R.sub.t I.sub.s. Upon occurrence of an internal fault, the quantities I.sub.p and R.sub.t I.sub.s are no longer equal. Accordingly, a transformer differential relay continuously compares the magnitude of the quantities I.sub.p and R.sub.t I.sub.s and will generate a trip signal when the difference between the quantities I.sub.p and R.sub.t I.sub.s exceeds a predetermined maximum. The trip signal is used to operate one or more circuit breakers in order to disconnect the input and/or the output of the faulted transformer from the power distribution system.
Although transformer differential relays work well to detect the occurrence of internal faults, they have been known to erroneously generate a trip signal as a result of transformer inrush current. Transformer inrush current occurs as a result of the application of voltage to the input windings. Voltage is applied to the transformer input windings when, for example, the system is initially powered or following the clearance of a fault in the system. The presence of inrush current causes a difference to occur in the quantities I.sub.p and R.sub.t I.sub.s. When the difference exceeds the predetermined maximum, occasionally the relay will generate a trip signal even though the difference was caused by inrush current and not by an internal fault. As a result, the reliability of the relay and consequently the reliability of the entire AC power distribution system incorporating such a relay is diminished.
Therefore, it is an object of the present invention to provide a method and apparatus for enhancing the reliability of transformer differential relays.
It is another object of the present invention to provide a method and apparatus which enable transformer differential relays to provide better discrimination between transformer current differentials due to transformer inrush current and current differentials due to the occurrence of a fault.
It is yet another object of the present invention to provide a method and apparatus which enable transformer differential relays to prevent the occurrence of a trip signal based solely on inrush current having a magnitude less than a predetermined value.
It is still another object of the present invention to provide a method and apparatus for generating additional trip signal restraint in a transformer differential relay upon occurrence of an external fault.
It is a further object of the present invention to provide a method and apparatus for increasing the reliability of the transformer differential relay in the presence of current transformer saturation on either internal or external faults.
It is yet a further object of the present invention to provide a method and apparatus for generating an additional trip signal operate quantity in a transformer differential relay when transformer differential current exceeds a predetermined maximum value.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following invention.